1. Field of the Invention
The present invention relates to printing plates using the indirect electrophotographic process. In particular, it relates to printing plates using the indirect electrophotographic process, which can reduce toner scattering, i.e., unwanted background images formed by toner particles, and prevent blur around toner images without reducing toner image density upon image formation.
2. Related Art
With recent developments of office work equipments and office automation, printing techniques utilizing printing plates using the indirect electrophotographic process of which plate making is easily accomplished by electrophotography, laser beam printers or the like are rapidly spreading in light printing field.
In particular, techniques where computer data or the like are directly outputted to printing plate materials by means of electrostatic printing machines including laser beam printers do not require block copy making unlike electrophotography and therefore they are excellent in plate making speed and their cost. For this reason, demand for such techniques are increasing late years.
Such electrostatic printing machines use dry toners for transferring and fixing images on surfaces of printing plates and, upon printing, surface portions of the plates deposited with toner receive lipophilic printing ink, i.e., they are made into image areas. Printing plates carrying image areas so formed are then subjected to a desensitization treatment with an etching solution and used as printing masters in lithography.
As such printing plates using the indirect electrophotographic process as described above, various materials such as those having an image receiving layer containing zinc oxide and formed on a water-resistant support have been proposed.
However, conventional printing plates using the indirect electrophotographic process suffer scattering of toner in non-image areas, i.e., unwanted background images formed by toner particles, upon toner image formation during plate making. Even if such unwanted background images formed by toner particles are not recognizable in images printed by laser beam printers, small amount of toner particles may carry ink when the plates are used for printing and may cause serious contamination to such an extent that commercial value of resulting printed matter is deteriorated.
Printing plates utilizing strongly hydrophilic resins such as carboxymethyl cellulose as a binder of image receiving layer have been known as materials which may solve the problem of unwanted background images formed by toner particles. However, since surface hardness of the image receiving layers utilizing such a binder is not so good and therefore printing durability is reduced.
To improve printing durability, printing plates utilizing polyvalent metal salts as a hardening agent together with hydrophilic binders have also been proposed. However, these printing plates have a drawback that they show reduced efficiency of toner transfer upon printing by electrostatic transfer techniques, since surface resistivity of their image receiving layers is lowered.
To solve the problem of unwanted background images formed by toner particles, also proposed is the method wherein printing plates are subjected to a desensitization treatment with an etching solution after toner image formation on the plates and non-image areas of the plates are treated with a special emulsion to eliminate unwanted background images formed by toner particles (Japanese Patent Application Laid-open KOKAI! No. 4-320844). However, this method is not so preferred because it requires further specific treatments after the etching treatment, which may lower operability and increase the process cost.
In addition, toner images formed by output of laser beam printers often exhibit blur around the images and resulting printing plates obtained from such images cannot provide clear toner images.
Apart from these problems, paper, plastice films or the like have been used for the conventional substrate of the printing plates using the indirect electrophotographic process.
However, paper, despite of its low cost, has problematic durability. Plastic films has durability but deformes at a relatively low temperature. For example, since the most common polyester film deforms at about 120.degree. to 150.degree. C., the plastic films begins to soften and deform when it is heated over that temperature. Accordingly, in case that a printing plate material having a plastic film as its substrate is formed to a printing plate by PPC copier, the plastic film is deformed by fusing heat and thereby the printing plate is out-put with severely waving. This phenomenon is particular when using the machine whose fusing temperature is high or that whose fusing time is long, such as laser beam printer (referred as LBP hereinafter) and it causes a jamming of the printing plate on the pass inside the machine.